The present invention relates to an ultra high frequency circuit improved by reduction of the parasite capacities which are formed between metallizations carried by the two main faces of the substrate of the circuit. Not only the ultra high frequency circuits constructed in hybrid form on a substrate formed by a ceramic wafer but also those constructed in integrated form on a substrate formed by a semi conductor material wafer which has been made semi insulating, such as GaAs for example, are concerned by the invention.
All ultra high frequency circuits include, whatever the functions they fulfill, a certain number of common elements. They are formed on a substrate made from a dielectric material having, on the first main face, active components-diodes, transistors, integrated circuits-conducting tracks in the form of microstrips and metallizations which serve as cabling relays or impedance adjusting segments for example. Active components are fixed to the substrate by means of metallized areas. Moreover, the second main face of the substrate is totally metallized so as to form a first ground plane for the ultra high frequency circuit. This metallization is further required for generating fixed impedance lines formed by a microstrip parallel to a ground plane and separated therefrom by a dielectric. It serves finally for fixing the ultra high frequency circuit on a support or metal case which also forms a second ground plane.
On the other hand, the metallization areas -other than the microstrips- and the metallized segments form with the ground plane parasite capacities of which the substrate forms the dielectric The parasite capacities have negligible values under static or DC current conditions but are unacceptable under ultra high frequency conditions, since they detune the circuit.